1. Field of the Invention
This invention relates to improvements in sprayable roof coating systems and, more particularly, to improvements in providing sprayed roof coatings of the type which include a latex-emulsion binder.
2. Description of the Prior Art
As is well-known, emulsions are intimate mixtures of two immiscible liquids, one of them being dispersed in the other in the form of fine droplets. A mixture of two miscible liquids, water and alcohol, for example, will not produce an emulsion, but a more intimate degree of dispersion, a solution. On the other hand, it is not possible to form an emulsion with two immiscible liquids alone, since such a system would lack stability. When water is mixed with toluene, for instance, the two liquids will quickly separate.
The conditions change completely when a dilute soap solution, instead of straight water, is mixed with an oil. A milky liquid develops and remains stable for a considerable length of time, forming a typical emulsion. Thus three components are necessary to produce emulsions--two immiscible liquids and an emulsifying agent or emulsifier to create the emulsion and to keep it stable.
To the casual observer emulsions appear as uniform, opaque liquids or pastes of white or slightly yellowish color. The microscope reveals that emulsions are by no means uniform substances. They are non-uniform and consist of a multitude of small droplets or particles, usually of spherical shape, and varying diameter, floating in the surrounding liquid. Emulsions with particles of large diameter are called coarse emulsions, and those with small particles are fine emulsions. The particles and the liquids in which they float are referred to as the phases of an emulsions. The particles are the discontinuous phase, and the medium which is the dispersion liquid is called the continuous phase. Also, since the droplets of the discontinuous phase are enclosed from all sides, it is also called the internal phase, and the continuous phase is also called the external phase.
In most emulsions, one of the phases is water, or an aqueous solution containing salts, soluble organic material, colloids, etc. The continuous phase thus is also called the water phase. The discontinuous is often called the oil phase, even it does not consist of oil. Many substances constitute the oil phase, all having the important common property of insolubility in water. These substances include hydrocarbons, resins, waxes, nitrocellulose, alkyds, rubber, vinyls and acrylics. They are referred to as "oil" since they behave in emulsions much like oil.
The substances used in emulsions may be classified into two groups: those which enter the water phase, and those which go into the oil phase. The first group making up the aqueous portion must be water soluble, or show a certain affinity toward water. This group has the general name of hydrophilic substances. The other group of substances which go into the oil phase have no affinity for water but rather are attracted to oil or oil-like material. They are known as hydrophobic substances. Typical hydrophilic substances are water-soluble compounds, many metal salts, and substances containing a relatively large number of oxy- or hydroxyl groups. Typical hydrophobic substances are oils, fats, waxes, and all compounds containing mainly carbon with few or no polar groups. This difference is important in the selection of other materials which will be blended with the emulsion, such as pigments, fillers, cement, sand, and all other ingredients which may go into the making of a mastic or concrete or mortar or other material desired. The characteristic properties of an emulsion are dictated by the external phase. If water is the external phase (oil-in-water emulsion), the emulsion may be diluted with water and not with oil or organic solvents. The opposite is true with a (water-in-oil emulsion) emulsion, which must be thinned with organic solvents. If water is added, the viscosity will increase rather than decrease. Indeed, this is one way of visually distinguishing between the two types. Thinning with water will indicate an oil-in-water emulsion if viscosity decreases. An increase in viscosity would indicate a water-in-oil emulsion.
Many types of emulsifying agents are used in preparing emulsions. Chemically they belong to different classes of compounds including salts, acids, bases, and esters. Physically, they are substances of medium molecular weight (above 300), and the molecules are of elongated shape and belong to the mixed polar/non-polar type. These emulsifying agents determine the type of emulsion produced and the particle size of the dispersed droplets.
The emulsions generally used in mastics and cementitious compositions are of the oil-in-water type, and sometimes contain more than 50 percent water. They are nevertheless oil-in-water emulsions, and are stable in the presence of the various salts and oxides comprising the cement. Not all emulsions are compatible with cement, and to select an emulsion primarily intended for production of paint or adhesives to act as an admixture for a cementitious mortar would be an error.
Latex emulsion systems are frequently used as the binder in mastic and cementitious mixtures. The internal or disperse phase in a latex system is a polymeric material comprised of spherically-shaped particles with diameters in the range of 200-300 nm; and the external phase or dispersion medium is usually aqueous phase. As is mentioned hereinafter, such particles are usually kept from normally aggregating by electrostatic repulsion between such particles due to the presence of ionic charge at the surface of such particles.
Emulsion polymerization refers to a unique process employed for some radical chain polymerizations. It involves the polymerization of monomers which are in the form of emulsions. [The process is not similar to suspension polymerization, but is quite different in its mechanism and reaction characteristics.] Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs and the kind of initiator employed. Emulsion polymerization was first employed for the production of synthetic styrene-butadiene rubbers during the 1940's when the supplies of natural rubber were cut off during World War II. Conjugated dienes such as butadiene and isoprene are presently polymerized and copolymerized in large part by the emulsion process. In addition, emulsion polymerization is also used extensively for vinyl acetate, vinyl chloride, acrylates, methacrylates, and various copolymers of these monomers.
The emulsion polymerization process has several distinct advantages. The physical state of the emulsion system makes it easy to control the process. Thermal and viscosity problems are much less significant than in bulk polymerization. The products of emulsion polymerizations can in some instances be employed directly without further separations but with appropriate blending operations. Such applications involve coatings, finishes, floor polishes, and paints. Aside from the physical difference between the emulsion and other polymerization processes, there is one very significant kinetic difference. For the other processes, there is an inverse relationship between the polymerization rate and the polymer molecular weight. This drastically limits one's practical ability to make large changes in the molecular weight of a polymer. Large increases in molecular weight can only be make by decreasing the polymerization rate by lowering the initiator concentration or lowering the reaction temperature. Emulsion polymerization is a unique process in that it affords a means of increasing the polymer molecular weight without decreasing the polymerization rate. Because of a different reaction mechanism, emulsion polymerization has the advantage of being able to simultaneously attain both high molecular weights and high reaction rates. To a large extent, the molecular weight and polymerization rate can be varied independently of each other.
As is well-known, the main components of an emulsion polymerization system are the monomer(s), dispersing medium, emulsifying agent, and water-soluble initiator. The dispersing medium is the liquid in which the various components are dispersed in an emulsion state by means of the emulsifying agent. The dispersing medium is usually water. The emulsifying agent is a surfactant whose action is due to its having both hydrophilic and hydrophobic segments in its molecular structure. Various other components may also be present in the emulsion system.
Also, as is well-known, for ionic latex emulsions, the surface charges at each particle (provided by the surfactant) assist in keeping the emulsion dispersed because of the electrostatic repulsion between the particles. This repulsion may be from cationic or from anionic charges; and the ways of using appropriate surfactants to maximize the dispersion of the emulsion by appropriate electrostatic repulsion between the particles of the emulsion are well-known. Additionally, it is known that an ionic emulsion may be coagulated (through aggregation of the separate particles and polymerization) by the addition of surfactants or electrolyte with opposite charge to the ions used to stabilize the latex particles. Additionally, it is known that coatings which contain ionic latex emulsions as the binder should have a controlled pH for best dispersion, i.e., maintaining the ionic electrostatic repulsion. For example, if a cationic latex emulsion is used as a binder (with pigments, etc.) in a mastic, the mastic should be maintained with a high pH.
Also, typically, water-based latex roof mastics may be placed on roofs in several manners, e.g., troweled, sprayed, etc. For such roof mastics of low enough viscosity to be sprayed (typically by air spray gun), the cure time is long enough (many hours or days) that there is often a chance for rain while the water-based latex roof mastic is not yet cured. It is noted that even if the cure time for sprayed coatings would be drastically reduced, thus reducing the chances for rain damage, there would be introduced new problems such as maintaining good adhesion of the coating to the base.
Addition of water to sprayed roof coating by raindrops typically softens the mastic to the point where it runs and destroys the integrity of the roof coating. Thus, in rainy-weather areas or other areas where rain may come shortly, roof spraying with such latex-based coatings is avoided even where it is otherwise the best coating to use for the job in terms of economics, long life, efficiency, etc. Thus, there have existed needs for sprayable latex-based roof coating systems which may be used in wet weather.
Furthermore, there have existed needs for sprayable economical latex-based roof coatings which are exceptionally resilient and long-wearing, both for use in dry areas and for use in wet areas. The inclusion in latex-based roof coatings of the kinds of asphaltic and recycled-rubber ingredients which have been used to provide resilient and long-wearing properties on floors, roads, and walkways has not been in the past successfully accomplished. To economically use these materials in latex roof coating systems, spraying is necessary. Thus, there exists a need for such sprayable resilient and long-wearing latex-based roof coatings.
The usual spray systems used in roof coating systems do not work efficiently with such materials (i.e., latex-based coatings including asphaltic and recycled-rubber ingredients). One major problem is that the usual air spray systems used currently in roof coating tend to clog and spray inefficiently (or not at all) when spraying such materials including such asphaltic and recycled-rubber ingredients. Thus there exists a need in roof coating systems for spraying systems which overcome such disadvantages.